Some components do not have function_nd method

[thomasaarholt]

The purpose of this issue is to highlight that some components are missing a function_nd method, which represents low-hanging fruit in some cases, and slightly more difficult (and desired) fruit in other cases.

@ericpre added the component.function_nd method, which gracefully lets one calculate a component's data array for the entire navigation space. I'm personally very grateful for this addition as it simplifies some of my code.

There are still some components that miss this method. Most notably, the ScaleableFixedPattern and the EELSCLEdge. I think the former is easy to implement, but @ericpre did you have a go at implementing the EELSCLEdge component? When I tried to create a similar method while working on #1477, I had difficulty with one of the aspects of the EELS component (I think it was the fine structure).

All components based on the Expression component automatically gain the function_nd method. The remaining components are listed below:

• Arctan: Implemented Arctan + EELSArctan as expressions #2395
• EELSCLEdge
• (EELS) Vignetting
• Heaviside: Convert heaviside step function component to expression #2262
• Logistic: Component logistic expression #2147.
• PESCoreLineShape Add more function_nd methods #2540
• PES_SEE Add more function_nd methods #2540
• ScalableFixedPattern Add more function_nd methods #2540
• Voigt: Voigt component as expression #2269

[thomasaarholt]

@ericpre this is not as required any longer for the linear fitting, at least with the two coming implementations. I think its still very nice to have though, for generating large numbers of varying components.